The sympathy strikers don't risk losing pay, since they strike against someone other than their employer. Laws and contracts don't seem to be preventing the unions from shutting down Tesla's operation. The need for production doesn't seem to be preventing this either.
So that people can get paid and produce goods and services for their society, obviously.
> If you're powerful enough to shut down a business, what's stopping you from getting whatever you want, regardless of whether it's reasonable?
Laws and contracts and as above the need for money and production, again, obviously.